The present paper reports on the role of water in plant pathology as possible stress factor and vector of pathogen. The latter aspect is considered in a scenario of general risk of introduction and spread of invasive plant pathogens. In addition to peculiar epidemiology aspects, the possible diagnostic methodologies and control methods are considered. The role of water as stress factor is analysed in a general frame of climatic global changes that could enhance the risk of severe drought events. Within this frame some model pathosystems are described where water plays a role as co-factor or inciting factor in disease development.